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<h2><span class="refentrytitle">Function reduce</span></h2>
<p>boost::mpi::reduce — Combine the values stored by each process into a single value at the root. </p>
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<h2 class="refsynopsisdiv-title">Synopsis</h2>
<div class="refsynopsisdiv"><pre class="synopsis"><span class="comment">// In header: &lt;<a class="link" href="../mpi/reference.html#doxygen.collectives_8hpp" title="Header &lt;boost/mpi/collectives.hpp&gt;">boost/mpi/collectives.hpp</a>&gt;

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<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> T<span class="special">,</span> <span class="keyword">typename</span> Op<span class="special">&gt;</span> 
  <span class="keyword">void</span> <span class="identifier">reduce</span><span class="special">(</span><span class="keyword">const</span> <a class="link" href="classboost_1_1mpi_1_1communicator.html" title="Class communicator">communicator</a> <span class="special">&amp;</span> comm<span class="special">,</span> <span class="keyword">const</span> <span class="identifier">T</span> <span class="special">&amp;</span> in_value<span class="special">,</span> <span class="identifier">T</span> <span class="special">&amp;</span> out_value<span class="special">,</span> 
              <span class="identifier">Op</span> op<span class="special">,</span> <span class="keyword">int</span> root<span class="special">)</span><span class="special">;</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> T<span class="special">,</span> <span class="keyword">typename</span> Op<span class="special">&gt;</span> 
  <span class="keyword">void</span> <span class="identifier">reduce</span><span class="special">(</span><span class="keyword">const</span> <a class="link" href="classboost_1_1mpi_1_1communicator.html" title="Class communicator">communicator</a> <span class="special">&amp;</span> comm<span class="special">,</span> <span class="keyword">const</span> <span class="identifier">T</span> <span class="special">&amp;</span> in_value<span class="special">,</span> <span class="identifier">Op</span> op<span class="special">,</span> <span class="keyword">int</span> root<span class="special">)</span><span class="special">;</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> T<span class="special">,</span> <span class="keyword">typename</span> Op<span class="special">&gt;</span> 
  <span class="keyword">void</span> <span class="identifier">reduce</span><span class="special">(</span><span class="keyword">const</span> <a class="link" href="classboost_1_1mpi_1_1communicator.html" title="Class communicator">communicator</a> <span class="special">&amp;</span> comm<span class="special">,</span> <span class="keyword">const</span> <span class="identifier">T</span> <span class="special">*</span> in_values<span class="special">,</span> <span class="keyword">int</span> n<span class="special">,</span> 
              <span class="identifier">T</span> <span class="special">*</span> out_values<span class="special">,</span> <span class="identifier">Op</span> op<span class="special">,</span> <span class="keyword">int</span> root<span class="special">)</span><span class="special">;</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> T<span class="special">,</span> <span class="keyword">typename</span> Op<span class="special">&gt;</span> 
  <span class="keyword">void</span> <span class="identifier">reduce</span><span class="special">(</span><span class="keyword">const</span> <a class="link" href="classboost_1_1mpi_1_1communicator.html" title="Class communicator">communicator</a> <span class="special">&amp;</span> comm<span class="special">,</span> <span class="keyword">const</span> <span class="identifier">T</span> <span class="special">*</span> in_values<span class="special">,</span> <span class="keyword">int</span> n<span class="special">,</span> <span class="identifier">Op</span> op<span class="special">,</span> 
              <span class="keyword">int</span> root<span class="special">)</span><span class="special">;</span></pre></div>
<div class="refsect1">
<a name="id-1.3.24.7.5.12.4"></a><h2>Description</h2>
<p><code class="computeroutput">reduce</code> is a collective algorithm that combines the values stored by each process into a single value at the <code class="computeroutput">root</code>. The values can be combined arbitrarily, specified via a function object. The type <code class="computeroutput">T</code> of the values may be any type that is serializable or has an associated MPI data type. One can think of this operation as a <code class="computeroutput">gather</code> to the <code class="computeroutput">root</code>, followed by an <code class="computeroutput">std::accumulate()</code> over the gathered values and using the operation <code class="computeroutput">op</code>.</p>
<p>When the type <code class="computeroutput">T</code> has an associated MPI data type, this routine invokes <code class="computeroutput">MPI_Reduce</code> to perform the reduction. If possible, built-in MPI operations will be used; otherwise, <code class="computeroutput"><a class="link" href="collectives_8hpp_1a1f25ce70bef33771218a8d47036b1ff5.html" title="Function reduce">reduce()</a></code> will create a custom MPI_Op for the call to MPI_Reduce.</p>
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<td><p><span class="term">Parameters:</span></p></td>
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<td><p><span class="term"><code class="computeroutput">comm</code></span></p></td>
<td><p>The communicator over which the reduction will occur.</p></td>
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<td><p><span class="term"><code class="computeroutput">in_value</code></span></p></td>
<td><p>The local value to be combined with the local values of every other process. For reducing arrays, <code class="computeroutput">in_values</code> contains a pointer to the local values. In this case, <code class="computeroutput">n</code> is the number of values that will be reduced. Reduction occurs independently for each of the <code class="computeroutput">n</code> values referenced by <code class="computeroutput">in_values</code>, e.g., calling reduce on an array of <code class="computeroutput">n</code> values is like calling <code class="computeroutput">reduce</code> <code class="computeroutput">n</code> separate times, one for each location in <code class="computeroutput">in_values</code> and <code class="computeroutput">out_values</code>.</p></td>
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<td><p><span class="term"><code class="computeroutput">out_value</code></span></p></td>
<td><p>Will receive the result of the reduction operation, but only for the <code class="computeroutput">root</code> process. Non-root processes may omit if parameter; if they choose to supply the parameter, it will be unchanged. For reducing arrays, <code class="computeroutput">out_values</code> contains a pointer to the storage for the output values.</p></td>
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<td><p><span class="term"><code class="computeroutput">op</code></span></p></td>
<td><p>The binary operation that combines two values of type <code class="computeroutput">T</code> into a third value of type <code class="computeroutput">T</code>. For types <code class="computeroutput">T</code> that has ssociated MPI data types, <code class="computeroutput">op</code> will either be translated into an <code class="computeroutput">MPI_Op</code> (via <code class="computeroutput">MPI_Op_create</code>) or, if possible, mapped directly to a built-in MPI operation. See <code class="computeroutput"><a class="link" href="structboost_1_1mpi_1_1is__mpi__op.html" title="Struct template is_mpi_op">is_mpi_op</a></code> in the <code class="computeroutput"><a class="link" href="../mpi/reference.html#doxygen.operations_8hpp" title="Header &lt;boost/mpi/operations.hpp&gt;">operations.hpp</a></code> header for more details on this mapping. For any non-built-in operation, commutativity will be determined by the <code class="computeroutput">is_commmutative</code> trait (also in <code class="computeroutput"><a class="link" href="../mpi/reference.html#doxygen.operations_8hpp" title="Header &lt;boost/mpi/operations.hpp&gt;">operations.hpp</a></code>): users are encouraged to mark commutative operations as such, because it gives the implementation additional lattitude to optimize the reduction operation.</p></td>
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<td><p><span class="term"><code class="computeroutput">root</code></span></p></td>
<td><p>The process ID number that will receive the final, combined value. This value must be the same on all processes. </p></td>
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<div class="copyright-footer">Copyright © 2005-2007 Douglas Gregor,
      Matthias Troyer, Trustees of Indiana University<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at &lt;ulink url="http://www.boost.org/LICENSE_1_0.txt"&gt;
        http://www.boost.org/LICENSE_1_0.txt &lt;/ulink&gt;)
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